1. Field of the Invention
The present invention relates to an image forming apparatus using powdered toner.
2. Description of the Prior Art
It is known how to provide an image forming method using an ink jet printer for word processors, facsimile machines, computers, etc.
The ink jet printer is a typical example of the so-called nonimpact printer. In the ink jet printer, pressure is applied to a prescribed liquid ink by means of a piezoelectric element or the like which produces ultrasonic vibrations so that the liquid ink is discharged from an ink nozzle into a prescribed electric field, the ink droplets being controlled by the electric field and deposited onto a recording paper to form an image thereon. Such an ink jet printing method has the advantage that a clear image can be formed without generating noise during the formation of the image. On the other hand, this method involves disadvantages in that it requires the use of a special kind of paper with surface treatment, etc. in order to control the speed at which the ink penetrates into the recording paper and also in that the nozzle tends to become clogged with foreign matters and other particles contained in the ink as the ink is supplied through the nozzle.
To overcome the above problems with the ink jet printer, an image forming apparatus using powdered toner as an image recording medium has been disclosed, for example, in Japanese Laid-Open Patent Publication No. 62-176873. This image forming apparatus is provided with a toner control means which performs control in such a manner that particles of powdered toner charged with a prescribed polarity are made to pass through pinhole-like toner passage holes by means of an electrostatic attraction force generated in response to an image output signal. The toner control means works to selectively apply toner particles to the recording paper to form the required image thereon. In such an apparatus, plain paper can be used as the recording paper, and also, since fine powdered toner is used, the toner passage holes are prevented from becoming clogged.
The toner control means for controlling the distribution of the powdered toner includes a board having numerous pinhole-like toner passage holes and a pair of electrodes insulated from each other disposed across the board so as to form an electric field within each toner passage hole. The paired electrodes are each provided with pinhole-like throughholes of the same size as that of the toner passage holes and are mounted on the respective sides of the board in such a manner that the throughholes are aligned with the toner passage holes. When a prescribed voltage is applied between the two electrodes and an electric field is formed along a prescribed direction within the corresponding toner passage hole, toner particles are made to pass through the toner passage hole. On the other hand, when toner particles are to be prevented from passing through the toner passage hole, a prescribed voltage is applied between the two electrodes in such a manner that an electric field is formed along the direction opposite to that of the electric field formed to allow toner particles to pass through the toner passage hole. The toner passed through the toner passage hole in the insulating board is deposited on the recording paper, placed suitably spaced apart from the insulating board, to form a dot thereon. Numerous toner dots combine to form an image on the recording paper.
The board in which the numerous pinhole-like toner passage holes are formed is usually made of plastic, while the electrodes are formed by bonding aluminum sheets having numerous pinhole-like through-holes in such a manner that the throughholes are aligned with the toner passage holes formed in the insulating board.
In recent years, there has been a demand for an image forming apparatus capable of forming high quality images, and therefore, there has arisen a need to make toner passage holes of a smaller diameter. Using toner passage holes of a smaller diameter, the resulting dots can be formed with a smaller diameter. However, when the toner passage holes are made smaller in diameter, there arises a possibility that the toner passage holes may become clogged with toner. To prevent this, the insulating board may be reduced in thickness, thereby reducing the axial length of each toner passage hole.
However, since a pair of electrodes are provided on the insulating board in order to form a prescribed electric field in each toner passage hole, an insulating board in which numerous toner passage holes are formed is required to have a prescribed dielectric strength as well as a mechanical strength. It is therefore not an easy job to reduce the thickness of the insulating board.
Furthermore, in such an image forming apparatus, ultrasonic vibrations are applied to the insulating board, in which case the pinhole-like toner passage holes formed in the insulating board can be prevented from becoming clogged. However, in the case of toner passage holes of extremely small diameter, it is not possible to completely prevent them from becoming clogged with toner.
Also, when the insulating board to which ultrasonic vibrations are applied is made of plastic, there is another problem that the ultrasonic vibrations may not be sufficiently transferred through the board. Therefore, in the case of a plastic board, there is a possibility that lumps of toner supplied may not be sufficiently broken up into fine particles. If the lumps of toner are not sufficiently broken up, the pinhole-like toner passage holes will become clogged with toner. Even if the ultrasonic vibrations are sufficiently transferred, fine particles of toner may adhere to the pinhole-like toner passage holes over a long period of use of the image forming apparatus, eventually causing the toner passage holes to be clogged with toner. When the toner passage holes are clogged with toner, the image quality will be substantially degraded, causing such troubles as white spots in the image formed on the recording paper.
Furthermore, there is the problem that it is not easy to bond the electrodes formed from aluminum sheets to the insulating board in such a manner that the pinhole-like throughholes formed in the aluminum sheets are aligned with the pinhole-like toner passage holes formed in the insulating board.
Also, since the recording paper onto which the toner is to be applied is placed at a suitable distance from the insulating board, there is a possibility that the toner passed through the toner passage holes in the board may radially spread before reaching the recording paper. If this happens, the toner passed through each toner passage hole adheres to the recording paper taking a larger area than the diameter of the toner passage hole and thus interfering with the formation of a clear image on the recording paper.
Normally, an image with clearer edges can be formed on the recording paper when the dot diameter of toner is smaller. Therefore, in the case of an image requiring production with clearer edges, such as characters, a smaller dot diameter is desirable. On the other hand, with a larger dot diameter, gradations of tone can be given to the edges of an image formed on the recording paper; therefore, in the case of an image having gradations in tone, such as photographs, a larger dot diameter is desirable. However, since the toner passage holes formed in the board each have a fixed bore diameter, the dot diameter cannot be adjusted, and therefore, it is not possible to adjust the image quality according to the image to be formed.
The toner supplied to the toner control means is usually charged with a prescribed polarity. However, when toner particles charged with opposite polarity are contained in the toner supplied to the toner control means, the toner particles of the opposite polarity are moved in the opposite direction from the toner charged with the prescribed polarity by the toner control means. As a result, the toner particles charged with opposite polarity are passed through the toner passage holes and adhere to the recording paper when it is not necessary to form an image. This will result in the fogging of the image formed on the recording paper.
Further, the amount of toner passing through each toner passage hole is usually fixed. If the image density is to be adjusted, digital processing such as the dither method will be required, which will complicate the construction of the image forming apparatus. Therefore, the problem is that the image density cannot be changed easily when there occurs a variation in the image density due to changes in the environmental conditions such as temperature, humidity, etc.
Usually, the numerous toner passage holes are arranged in arrays each extending substantially perpendicular to the advancing direction of the recording paper. Therefore, in order to form a continuous line or an image having a substantial area, it is necessary to form the dots overlapping each other. Each individual dot is formed by the toner passed through each toner passage hole, while on the other hand, the toner passage holes are arranged suitably spaced apart from each other. Therefore, in order to form adjacent dots overlapping each other, the dot diameter must be made larger than the diameter of each toner passage hole. To make the dot diameter larger, the amount of toner passed through the toner passage hole must be increased. This requires decreasing the recording paper transport speed, which causes the problem that the image forming speed is decreased. Furthermore, when the dot diameter is made larger, the density in the periphery of the dot decreases, preventing the formation of an image with sharp edges.